/Engine/Source/Runtime/Slate/Public/Widgets/Views/STreeView.h
Given: an array of items (ItemsSource) each of which potentially has a child. Task: populate the LinearizedItems array with a flattened version of the visible data items. In the process, remove any items that are not visible while maintaining any collapsed items that may have expanded children.
true if we encountered expanded children; false otherwise.
bool PopulateLinearizedItems ( [TArrayView](API\Runtime\Core\Containers\TArrayView)< const ItemType > InItemsSource, [TArray](API\Runtime\Core\Containers\TArray)< ItemType > & InLinearizedItems, [TArray](API\Runtime\Core\Containers\TArray)< [FItemInfo](API\Runtime\Slate\Widgets\Views\FItemInfo) > & NewDenseItemInfos, [TBitArray](API\Runtime\Core\Containers\TBitArray)<> NeedsParentWire, [TItemSet](API\Editor\UnrealEd\Editor\SListView\TItemSet) & OutNewSelectedItems, [TSparseItemMap](API\Editor\UnrealEd\STreeView\TSparseItemMap) & NewSparseItemInfo, bool bAddingItems, int32 ParentIndex )